The processes by which organs acquire global structures and patterns during development are highly complex, and likely involve multiple, overlapping biochemical pathways. In the vertebrate heart, for example, the first key visible step in this process is chamber morphogenesis, involving the fashioning of the atrium and the ventricle. Proper orientation of these two functionally distinct contractile units is required for unidirectional blood flow, which begins with the first heartbeat of an organism. Properly formed chambers thereafter are the substrates upon which further heart development is superimposed.
Over recent years, much has been learned about the molecular mechanisms that are responsible for the acquisition of characteristic atrial and ventricular cell fates (Fishman et al., Development 124:2099-2117, 1997; Srivastava et al., Nature 407:221-226, 2000). However, both embryological and molecular steps that fashion the higher order structures of these chambers have proven to be more elusive because, in part, unlike cell fate decisions, these steps can be studied meaningfully only in living organisms. The zebrafish, Danio rerio, is a convenient organism to use in genetic and biochemical analyses of development. It has an accessible and transparent embryo, allowing direct observation of organ function from the earliest stages of development, has a short generation time, and is fecund.